my 92 x must be a freak of nature,this past weekend i drove to seaside,oregon on the coast,FROM bend oregon,little under 300 mile one way trip for me,even going over 2 mountain passes,i still averaged 22mpg
i filled up in longview washington,and drove down to albany oregon{99% flat roads and highway speeds the whole way} i got 24 mpg.
around the city of longview{which is sea level} i was getting about 18 mpg average for city driving,around my town{bend oregon} i average around 15mpg city,and around flat 20s for highway,mostly due to the higher elevation. my x only has 92k miles on it though.
edit,mine is also 100% bone stock